While reading on block ciphers and DES I read that two-round Feistel network is not a secure PRP? Is there any easy to understand proof to explain the intuition behind this statement. I did search around and also reviewed this question but wasn't really able to understand why it isn't secure PRP. Any help would be much appreciated!
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$\begingroup$ The key-word is Luby and Rackoff construction $\endgroup$– kelalakaOct 25, 2020 at 9:07
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$\begingroup$ Yes I read that and it didn't make sense to me maybe because of the notations used here: crypto.stanford.edu/pbc/notes/crypto/prp.html Is there an easy to understand explanation like an example or proof? $\endgroup$– AlexOct 25, 2020 at 9:10
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1$\begingroup$ Q: Luby-Rackoff theorem confusion and and search $\endgroup$– kelalakaOct 25, 2020 at 11:02
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I read that two-round Feistel network is not a secure PRP
That's easily seen:
It holds $P_L\oplus C_L=F_0(P_R)$. That implies a distinguishable property: for any fixed $P_R$ and whatever the round function $F_0$, when we flip bit(s) in $P_L$, that flips the corresponding bit(s) in $C_L$ and leaves the other bit(s) in $C_L$ unchanged.
That property allows a break under Chosen Plaintext Attack.
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$\begingroup$ Perfect simple explanation that I was looking for! Thanks a lot! $\endgroup$– AlexOct 25, 2020 at 20:51
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$\begingroup$ @hola: the common way of counting rounds in Feistel ciphers is counting how many round functions there are. In the picture, there are two. $\endgroup$– fgrieu ♦May 30, 2021 at 7:46